The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Aqueous solutions receive a lot of attention as water covers over seventy percent of the planet and is vital for all known forms of life. In water, hydrogen atoms constantly transfer from one water molecule to another forming two charged species: the hydroxide ion (OH−) and the hydronium ion (H3O+). While the relative concentrations of these two ions can vary, their product remains constant. Addition of foreign species, such as an acid or a base, can adjust the ratio of these two ions. The pH scale, a log concentration scale, has been developed to define such solutions, which is related to the concentration of hydronium ions by pH=−log [H3O+]. The pH of a solution can have a significant effect on chemical processes, therefore both the measurement and control of pH is important for materials, life, and environmental sciences. For example, it would be beneficial to monitor the pH inside the human body. However, such monitoring is very difficult with conventional technologies. Additionally, pH sensors may be exposed to extraordinary conditions, such as extreme temperature and pressure, in applications such as geochemistry for monitoring pH levels inside the earth's ocean to facilitate carbon sequestration technologies. Once again, such monitoring is very difficult with conventional technologies.
The most common pH sensors are glass electrodes with a salt solution-filled glass membrane limiting their applications. Some common problems associated with such electrodes include temperature dependence and errors in measurement in intense conditions (i.e., low pH and low ionic-strength solutions).1 Additionally, glass pH electrodes become sensitive to alkali-metal ions at high pH, degrade if dehydrated, and require calibration with a standard buffer (potentially introducing associated errors).
The field of ion-selective field-effect transistors (ISFETs) which started more than 40 years ago has promised development of rugged, small, rapid response pH sensor devices. Additionally, ISFETs would not require hydration and would be inert toward harsh environments. While there are numerous advantages of using ISFETs, one major limitation of the technology involves the requirement of a reference electrode, ultimately limiting the ability to reduce their size.